The present invention relates to an electron beam irradiation process for irradiating an object with an electron beam (EB) which is obtained by accelerating electrons with a voltage applied thereto in a vacuum and guiding the accelerated electrons into a normal-pressure atmosphere, and to an object irradiated with such an electron beam.
There has been proposed a process utilizing electron beam irradiation to crosslink, cure or modify a coating material applied to a substrate or base, such as paint, printing ink, adhesive, pressure sensitive, etc., or other resin products, and extensive studies have been made up to the present. In this process, electrons are accelerated with a voltage applied thereto in a vacuum and the accelerated electrons are guided into a normal-pressure atmosphere, such as in the air, so that an object may be irradiated with an electron beam (EB).
Crosslinking, curing or modification by means of electron beam irradiation have the following advantages:
(1) Organic solvent need not be contained as a diluent, and thus the adverse effect on the environment is small.
(2) The rate of crosslinking, curing or modification is high (productivity is high).
(3) The area required for crosslinking, curing or modification is small, compared with heat drying treatment.
(4) The substrate or base is not applied with heat (electron beam irradiation is applicable to those materials which are easily affected by heat).
(5) Post-treatment can be immediately carried out (cooling, aging, etc. are unnecessary).
(6) It is necessary that the conditions for electrical operation be controlled, but the required control is easier than the temperature control for heat drying treatment.
(7) Neither initiator nor sensitizing agent is required, and thus the final product contains less impurities (quality is improved).
According to conventional electron beam irradiation techniques, however, a high-energy electron beam is used to crosslink, cure or modify objects at a high rate, and no consideration is given to energy efficiency.
Conventional techniques are also associated with problems such as the problem that much initial investment is required because of large-sized apparatus, the problem that inerting. by means of an inert gas such as nitrogen, which is high in running cost, is needed in order to eliminate inhibition to the reaction at surface caused due to generation of oxygen radical, and the problem that shielding from secondary X-ray is required.
Specifically, conventional electron beam curing or crosslinking uses an acceleration voltage which is usually as high as 200 kV to 1 MV and thus X-rays are generated, making it necessary to provide a large-scale shield for the apparatus. Also, where such a high-energy electron beam is used, care must be given to possible adverse influence on the working environment due to generation of ozone. Since the reaction at the surface of an object is inhibited due to generation of oxygen radical, moreover, inerting by means of an inert gas such as nitrogen is required.
Further, an electron beam generated with a high acceleration voltage applied thereto penetrates to a great depth and thus can sometimes deteriorate the substrate or base such as a resin film or paper. In the case of paper, for example, disintegration of cellulose due to the breakage of glycoside bond takes place at a relatively small dose, and it is known that deterioration in the folding strength is noticeable even at an irradiation dose of 1 Mrad or less. Especially in the case where the substrate or base has a coating material (printing ink, paint, adhesive, etc.) of 0.01 to 30 xcexcm thick printed thereon or applied thereto, the thickness of the coating material is small and the substrate or base may have an exposed surface having no coating material thereon, often giving rise to a problem that the substrate or base is deteriorated.
Accordingly, there is a demand for low-energy electron beam irradiation apparatus and process which use low acceleration voltage and which permit reduction in size of the apparatus.
To meet the demand, various apparatus and process using low acceleration voltage for electron beam irradiation have been proposed, and Japanese Patent Disclosure (KOKAI) No. 5-77862, for example, discloses a process for 30-Mrad irradiation at 200 kV, as an example of electron beam irradiation at a low acceleration voltage. However, even with this process, the acceleration voltage is not low enough to prevent deterioration of the substrate or base and also inerting is required.
Japanese Patent Disclosure No. 6-317700 discloses an apparatus and process for irradiating an electron beam with the acceleration voltage adjusted to 90 to 150 kV. According to this technique, a titanium or aluminum foil of 10 to 30 xcexcm in thickness is used as a window material which intervenes between an electron beam generating section of the electron beam irradiation apparatus, in which electrons released from the cathode are guided and accelerated to obtain an electron beam, and an irradiation room in which an object is irradiated with the electron beam.
However, even with this technique, when the acceleration voltage is set to 100 kV or less in actuality, the penetrating power of the electron beam is very low, and since most of the electron beam is absorbed by the window material, the electron beam cannot be efficiently guided into the irradiation room. Also, the temperature of the window material may possibly rise up to its heat resistance temperature or higher. Consequently, the apparatus is in practice used with the acceleration voltage set at a level higher than 100 kV, and even with such acceleration voltage, deterioration of the substrate or base can be caused.
Thus, the electron beam curing technique has been attracting attention as a process which serves to save energy, does not require the use of solvent and is less harmful to the environment, but it cannot be said that the technique has been put to fully practical use because of the aforementioned problems.
The present invention was created in view of the above circumstances, and an object thereof is to provide an electron beam irradiation process capable of irradiating an electron beam with high energy efficiency and an object irradiated with such an electron beam, without entailing problems with apparatus etc.
According to a first aspect of the present invention, there is provided an electron beam irradiation process for performing electron beam irradiation by using a vacuum tube-type electron beam irradiation apparatus, wherein an object is irradiated with an electron beam with an acceleration voltage for generating the electron beam set at a value smaller than 100 kV. Also, according to this aspect of the invention, an electron beam irradiation process is provided wherein the acceleration voltage is 10 to 60 kV and the object comprises a coating of 0.01 to 30 xcexcm thick formed on a substrate or base.
According to a second aspect of the present invention, an electron beam irradiation process for irradiating an object with an electron beam is provided, wherein an electron beam is irradiated in such a manner that a rate of absorption y (%) of the irradiated electron beam by an object, which rate of absorption is expressed as xe2x80x9cabsorbed dose for a certain depth/all absorbed dosexe2x80x9d, fulfills a relationship indicated by expression (1) below, where x is a product of penetration depth (xcexcm) and specific gravity of the object. Also provided according to this aspect of the invention is an electron beam irradiation process wherein an acceleration voltage for generating the electron beam is 100 kV or less and the object has a thickness of 50 xcexcm or less. Further, an electron beam irradiation process is provided wherein irradiation of the electron beam is performed using a vacuum tube-type electron beam irradiation apparatus.
y24 xe2x88x920.01x2+2x(0 less than xxe2x89xa7100)xe2x80x83xe2x80x83(1)
The penetration depth indicates a distance in the thickness direction of the object for which the irradiated electron beam penetrates.
According to a third aspect of the present invention, there is provided an electron beam irradiation process for irradiating an object with an electron beam, wherein when an acceleration voltage of an electron beam to be irradiated is lower than or equal to 40 kV, the electron beam is irradiated in such a manner that an oxygen concentration of a region irradiated with the electron beam is substantially equal to or lower than air, and when the acceleration voltage of an electron beam to be irradiated is higher than 40 kV, the electron beam is irradiated in such a manner that the oxygen concentration of the region irradiated with the electron beam fulfills a relationship indicated by expression (a)
Yxe2x89xa61.19xc3x97102xc3x97exp(xe2x88x924.45xc3x9710xe2x88x922xc3x97X)xe2x80x83xe2x80x83(a)
where X is the acceleration voltage (kV) and Y is the oxygen concentration (%) of the region irradiated with the electron beam.
Preferably, in this case, when an acceleration voltage of an electron beam to be irradiated is lower than or equal to 40 kV, the electron beam is irradiated in such a manner that an oxygen concentration of a region irradiated with the electron beam is substantially equal to or lower than air, and when the acceleration voltage of an electron beam to be irradiated is higher than 40 kV, the electron beam is irradiated in such a manner that the oxygen concentration of the region irradiated with the electron beam fulfills a relationship indicated by expression (b)
1.19xc3x97102xc3x97exp(xe2x88x924.45xc3x9710xe2x88x922xc3x97X)xe2x89xa7Yxe2x89xa70.05xe2x80x83xe2x80x83(b)
where X is the acceleration voltage (kV) and Y is the oxygen concentration (%) of the region irradiated with the electron beam.
According to a fourth aspect of the present invention, there is provided an electron beam irradiation process, wherein an object having a curved or uneven surface is irradiated with an electron beam while an electron beam generating section of an electron beam irradiation apparatus is moved for scanning. Also, according to this aspect of the invention, an electron beam irradiation process is provided wherein the electron beam generating section is moved for scanning while a distance between the electron beam generating section and the object is kept at a constant value by means of a sensor.
According to a fifth aspect of the present invention, there is provided an electron beam irradiation process, wherein a distribution of degree of crosslinking, curing or modification is created in a thickness direction of an object by irradiating the object with an electron beam.